Method for removing DNA contaminants from a protein-containing sample

ABSTRACT

A method is provided for purifying physiologically active proteins, especially antibodies, in order to remove impurities such as DNA contaminants and viruses with minimal loss of physiologically active proteins. The physiologically active protein is introduced into an aqueous solution of low conductivity at a pH of below the isoelectric point of the physiologically active protein to precipitate impurities as particles. The particles are removed, leaving a purified physiologically active protein.

TECHNICAL FIELD

The present invention relates to a method for purifying proteins, morespecifically to a method for removing impurities such as DNAcontaminants from a sample containing a physiologically active proteinsuch as antibody molecules.

BACKGROUND ART

Advances in gene recombinant technology have enabled a stable supply ofvarious protein formulations. In particular, a variety of recombinantantibody drugs, which are more selective than normal drugs, have beendeveloped and entered clinical trial in recent years.

In these recombinantly-produced physiologically activeprotein-containing formulations, there is a need to remove host DNA andimpurities (e.g., DNA contaminants) associated with viral contamination.Under present World Health Organization (WHO) criteria, the amount ofDNA in biological drugs should not exceed 100 pg DNA/dose. To meet thiscriteria, in general, an aqueous medium containing host cell-derivedphysiologically active proteins is treated by anion-exchangechromatography, hydroxyapatite chromatography or a combination thereof,for the purpose of removing DNA.

In particular, in a case where a physiologically active protein is anantibody produced recombinantly in mammalian host cells, the aqueousmedium is treated by affinity column chromatography on Protein A or Gbefore being purified by various types of chromatography, based on thebinding property of Protein A or Protein G to IgG Fc chain.

By way of example, in JP KOHYO 5-504579, an antibody-containing aqueousmedium obtained from mammalian cell culture is subjected to Protein A/Gcolumn chromatography to adsorb antibody molecules onto the column,followed by elution with an acidic solution (about 0.1 M citric acid, pH3.0-3.5) to release the antibody molecules. The resulting acidic eluateis subjected sequentially to ion-exchange column chromatography and sizeexclusion column chromatography to give the purified antibody molecules.

However, these individual chromatographic processes and combinationsthereof are time-, labor- and cost-consuming, as well as beingcomplicated. Moreover, they fail to provide stable results.

Thus, the object of the present invention is to provide a simpler andless expensive method for purifying physiologically active proteins,especially antibodies, which can ensure removal of impurities such asDNA contaminants and viruses, and which can minimize a loss ofphysiologically active proteins.

Disclosure of the Invention

As a result of extensive and intensive efforts made to overcome theseproblems, the inventors of the present invention have made thesurprising finding that impurities such as DNA contaminants and virusescan be efficiently removed from a physiologically activeprotein-containing sample without using complicated chromatographicprocesses when the sample is formed into an aqueous solution of lowconductivity at a pH below the isoelectric point of the physiologicallyactive protein and then filtrated through a filter to remove theresulting particles. This finding led to the completion of the presentinvention.

Namely, the present invention provides the following.

-   (1) A method for removing impurities in a physiologically active    protein-containing sample, which comprises the steps of:

1) forming the physiologically active protein-containing sample into anaqueous solution of low conductivity having a pH equal to or lower thanthe isoelectric point of the physiologically active protein; and

2) removing the resulting particles.

-   (2) The method according to (1) above, wherein the aqueous solution    of low conductivity has a conductivity of 0 to 100 mM, as expressed    in molarity.-   (3) The method according to (1) or (2) above, wherein the aqueous    solution of low conductivity has an ionic strength of 0 to 0.2.-   (4) The method according to any one of (1) to (3) above, wherein the    aqueous solution of low conductivity has a conductivity of 0 to 300    mS/m.-   (5) The method according to any one of (1) to (4) above, wherein the    solution is selected from aqueous solutions of hydrochloric acid,    citric acid and acetic acid.-   (6) The method according to any one of (1) to (5) above, wherein the    pH of the aqueous solution is equal to or lower than the isoelectric    point of the physiologically active protein and equal to or higher    than pH 2.0.-   (7) The method according to any one of (1) to (6) above, wherein the    impurities are DNA contaminants.-   (8) The method according to any one of (1) to (6) above, wherein the    impurities are viruses.-   (9) The method according to (7) above, wherein the physiologically    active protein-containing sample has the DNA contaminants at a DNA    concentration of 22.5 pg/ml or less after the treatment of removal    of DNA contaminants.-   (10) The method according to any one of (1) to (9) above, wherein    the physiologically active protein is an antibody.-   (11) The method according to (10) above, wherein the antibody is an    IgG antibody.-   (12) The method according to (10) or (11) above, wherein the    antibody is a humanized monoclonal antibody.-   (13) The method according to (12) above, wherein the antibody is a    humanized anti-IL-6 receptor antibody.-   (14) The method according to (12) above, wherein the antibody is a    humanized anti-HM1.24 antigen monoclonal antibody.-   (15) The method according to (12) above, wherein the antibody is a    humanized anti-parathyroid hormone-related peptide antibody    (anti-PTHrP antibody).-   (16) The method according to any one of (1) to (9) above, wherein    the physiologically active protein is granulocyte colony-stimulating    factor.-   (17) The method according to any one of (1) to (16) above, wherein    the particles are removed by filtration through a filter.-   (18) The method according to (1) above, wherein step 1) is    accomplished by forming the physiologically active    protein-containing sample into an acidic or alkaline aqueous    solution of low conductivity, and adjusting the resulting sample    with a buffer to a pH equal to or lower than the isoelectric point    of the physiologically active protein.-   (19) The method according to (1) above,

wherein the physiologically active protein is an antibody, and

wherein step 1) is accomplished by subjecting the antibody-containingsample to affinity chromatography on Protein A or G, eluting the samplewith an acidic aqueous solution of low conductivity, and adjusting theresulting eluate with a buffer to a pH equal to or lower than theisoelectric point of the antibody.

-   (20) The method according to (18) or (19) above, wherein the buffer    is an aqueous solution of Tris.-   (21) A purified physiologically active protein obtainable by the    method according to any one of (1) to (20) above.-   (22) A method for manufacturing a medical protein formulation, which    comprises a purification step in which the method according to any    one of (1) to (20) above is used.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of a physiologically active protein contained in a sample to bepurified by the method of the present invention include, but are notlimited to, hematopoietic factors such as granulocyte colony-stimulatingfactor (G-CSF), granulocyte-macrophage colony-stimulating factor(GM-CSF), erythropoietin (EPO) and thrombopoietin, cytokines such asinterferons, IL-1 and IL-6, monoclonal antibodies, tissue plasminogenactivator (TPA), urokinase, serum albumin, blood coagulation factorVIII, leptin, insulin, and stem cell growth factor (SCF). Among theseproteins, preferred are G-CSF and antibodies including monoclonalantibodies, and more preferred are monoclonal antibodies. In anembodiment of the present invention using Protein A/G affinitychromatography, monoclonal antibodies are preferred for purification.Antibodies are categorized into IgG, IgA, IgE, IgD and IgM classes, withIgG antibodies being preferred.

The term “physiologically active protein” is intended to mean a proteinhaving substantially the same biological activities as a correspondingphysiologically active protein of mammalian (especially human) origin.Such a protein may either be native or genetically recombinant,preferably genetically recombinant. Genetically recombinantphysiologically active proteins may be prepared by production inbacterial cells such as E. coli; yeast cells; or animal-derived culturedcells such as Chinese hamster ovary (CHO) cells, C127 cells or COScells. The proteins thus prepared are isolated and purified in variousmanners before use. Such genetically recombinant proteins encompassthose having the same amino acid sequence as the corresponding nativeprotein, as well as those comprising deletion, substitution or additionof one or more amino acids in the amino acid sequence, but retaining thebiological activities mentioned above. Further, such proteins includethose chemically modified with PEG, etc.

When a physiologically active protein is a glycoprotein, it may havesugar chains of any origin, preferably of mammalian origin. Mammalianorigins include, for example, Chinese hamster ovary (CHO) cells, BHKcells, COS cells and human-derived cells, with CHO cells being mostpreferred.

When a physiologically active protein is EPO, it may be prepared in anymanner, for example, by obtaining from human urine in various manners orby producing with genetic engineering techniques in bacterial cells suchas E. coli, yeast cells, Chinese hamster ovary (CHO) cells, BHK cells,COS cells, human-derived cells or the like (e.g., as described in JPKOKAI 61-12288). EPO thus prepared is extracted, isolated, and purifiedin various manners before use. In addition, EPO may be chemicallymodified with PEG, etc. (see International Publication No. WO90/12874).EPO as used herein further includes those originally unglycosylated butchemically modified with PEG, etc. Likewise, EPO analogs are alsoincluded, which are modified to have at least one additional site forN-linked or O-linked glycosylation in the amino acid sequence of EPO(see, e.g., JP KOKAI 08-151398, JP KOHYO 08-506023). Instead ofincreasing the number of glycosylation sites, EPO analogs may also bemodified to have an increased content of sugar chains such as sialicacid for an increased amount of sugar chains.

When a physiologically active protein is G-CSF, any G-CSF can be used aslong as it is highly purified. G-CSF as used herein may be prepared inany manner, for example, by obtaining from cultured human tumor celllines or by producing with genetic engineering techniques in bacterialcells such as E. coli; yeast cells; or animal-derived cultured cellssuch as Chinese hamster ovary (CHO) cells, C127 cells or COS cells.G-CSF thus prepared is extracted, isolated, and purified in variousmanners before use. Preferred are those produced recombinantly in E.coli cells, yeast cells or CHO cells. The most preferred are thoseproduced recombinantly in CHO cells. In addition, G-CSF may bechemically modified with PEG, etc. (see International Publication No.WO90/12874).

When a physiologically active protein is a monoclonal antibody, it maybe prepared in any manner. In principle, a monoclonal antibody can beproduced using known techniques by immunizing a sensitizing antigen inaccordance with conventional procedures for immunization, fusing theresulting immunocytes with known parent cells through conventionalprocedures for cell fusion, and then screening monoclonalantibody-producing cells through conventional procedures for screening.

Alternatively, antibody genes are cloned from hybridomas, integratedinto appropriate vectors, and then transformed into hosts to produceantibody molecules using gene recombination technology. The geneticallyrecombinant antibodies thus produced may also be used in the presentinvention (see, e.g., Carl, A. K. Borrebaeck, James, W. Larrick,THERAPEUTIC MONOCLONAL ANTIBODIES, Published in the United Kingdom byMACMILLAN PUBLISHERS LTD, 1990). More specifically, cDNA of antibodyvariable domains (V domains) is synthesized from hybridoma mRNA usingreverse transcriptase. Upon obtaining DNA encoding the target antibody Vdomains, the DNA is ligated to DNA encoding desired antibody constantdomains (C domains) and integrated into an expression vector.Alternatively, the DNA encoding the antibody V domains may be integratedinto an expression vector carrying the DNA of the antibody C domains.The DNA construct is integrated into an expression vector such that itis expressed under control of an expression regulatory region, e.g., anenhancer or a promoter. Host cells are then transformed with thisexpression vector for antibody expression.

In the present invention, it is possible to use genetically recombinantantibodies (e.g., chimeric antibodies, humanized antibodies) that areartificially modified with a view to attenuating the characteristics asheteroantigen to human. These modified antibodies may be prepared in aknown manner. A chimeric antibody is composed of variable domains ofheavy and light chains from a non-human mammalian (e.g., mouse) antibodyand constant domains of heavy and light chains from a human antibody. Toobtain chimeric antibodies, DNAs encoding such mouse antibody variabledomains may be ligated to DNAs encoding the human antibody constantdomains, and then integrated into an expression vector, followed bytransformation into a host for antibody production.

Humanized antibodies are also called reshaped human antibodies and areobtained by grafting complementarity determining regions (CDRs) ofnon-human mammalian (e.g., mouse) antibodies to replace those of humanantibodies. Standard gene recombination procedures for this purpose arealso known. More specifically, a DNA sequence designed to allow ligationbetween CDRs of mouse antibody and framework regions (FRs) of humanantibody is synthesized by PCR from several oligonucleotides which areprepared to have sections overlapping with one another at the ends. TheDNA thus obtained is ligated to DNA encoding human antibody constantdomains, and integrated into an expression vector, followed bytransformation into a host for antibody production (see European PatentPublication No. EP 239400 and International Publication No. WO96/02576). The FRs of human antibody, which is ligated via CDRs, areselected such that the complementarity determining regions form afavorable antigen-binding site. If necessary, amino acid substitutionsmay be made in the framework regions of antibody variable domains suchthat the complementarity determining regions of reshaped humanizedantibody may form an appropriate antigen-binding site (Sato, K. et al.,Cancer Res. (1993) 53, 851-856).

A humanized anti-IL-6 receptor antibody (hPM-1) can be presented as apreferred example for such reshaped humanized antibodies (seeInternational Publication No. WO92-19759). In addition to this, ahumanized anti-HM1.24 antigen monoclonal antibody (see InternationalPublication No. WO98-14580), a humanized anti-parathyroidhormone-related peptide antibody (anti-PTHrP antibody; see InternationalPublication No. WO98-13388), a humanized anti-tissue factor antibody(see International Publication No. WO99-51743) and the like are alsopreferred for use in the present invention.

Procedures for obtaining human antibodies are also known. For example,human lymphocytes are sensitized in vitro with a desired antigen or adesired antigen-expressing cell, and the sensitized lymphocytes are thenfused with human myeloma cells (e.g., U266) to give desired humanantibodies having binding activity to the antigen (see JP KOKOKU01-59878). Alternatively, transgenic animals having the entirerepertories of human antibody genes may be immunized with an antigen toobtain desired human antibodies (see International Publication Nos. WO93/12227, WO 92/03918, WO 94/02602, WO 94/25585, WO 96/34096 and WO96/33735). There are additional techniques using human antibodylibraries to give human antibodies by panning. For example, humanantibody variable domains may each be expressed as a single-chainantibody (scFv) on the surface of phages by phage display technology,followed by selection of phages binding to the antigen. By analyzinggenes of the selected phages, it is possible to determine DNA sequencesencoding human antibody variable domains binding to the antigen. Oncethe DNA sequences of scFv binding to the antigen have been identified,the sequences may be used to construct appropriate expression vectors toobtain human antibodies. These techniques are already well known and canbe found in WO 92/01047, WO 92/20791, WO 93/06213, WO 93/11236, WO93/19172, WO 95/01438 and WO 95/15388.

Further, human antibodies produced in transgenic animals and the likeare also preferred.

Furthermore, the antibody as used herein encompasses antibody fragmentsincluding Fab, (Fab′)₂, Fc, Fc′ and Fd, as well as reshaped antibodiesincluding monovalent or polyvalent single chain antibodies (scFV).

As used herein, the term “physiologically active protein-containingsample” or an “antibody-containing sample” is preferably intended tomean a culture medium of mammalian cells (e.g., CHO cells) containingphysiologically active protein molecules or antibody molecules producedby culture, which may further be subjected to partial purification orother certain treatment(s).

In a preferred embodiment of the present invention, impurities in aphysiologically active protein-containing sample are removed by a methodcomprising the steps of:

1) forming the physiologically active protein-containing sample into anaqueous solution of low conductivity at a pH equal to or lower than theisoelectric point of the physiologically active protein; and

2) removing the resulting particles.

Any substance may be removed as an impurity by the method of the presentinvention as long as it is not a target protein to be purified. Examplesof such impurities include DNA contaminants, viruses, Protein A (elutedfrom columns), endotoxins, HCP (host cell-derived proteins), as well asmedium components Hy-Fish(FL) and IGF, with DNA contaminants or virusesbeing preferred. As used herein, the term “DNA contaminants” is intendedto mean DNA molecules present in a physiologically activeprotein-containing sample. Examples include host-derived DNAs andcontamination-derived viral DNAs.

There is no particular limitation on the type of virus to be removed bythe method of the present invention. Any virus, including DNA and RNAviruses, may be removed. Examples of RNA viruses include retroviruses(e.g., X-MuLV), reoviruses (e.g., Reo 3) and parvoviruses (e.g., MVM).Illustrative examples of viruses removed by the method of the presentinvention include, for example, X-MuLV, PRV, Reo 3, MVM, VSV, herpessimplex, CHV, Sindbis, mumps, vaccinia, Measle, Rubella, influenza,herpes zoster, cytomegalo, parainfluenza, EB, HIV, HA, HB, NANB, ATL,ECHO and parvovirus, with X-MuLV, Reo 3, MVM and PRV being preferred.

As used herein, the term “aqueous solution of low conductivity ” isgenerally intended to mean an aqueous solution which has a molarity of 0to 100 mM, preferably 0 to 50 mM, more preferably 0 to 30 mM, or whichhas an ionic strength of 0 to 0.2, preferably 0 to 0.12, or which has aconductivity of 0 to 300 mS/m, preferably 0 to 200 mS/m, more preferably0 to 150 mS/m.

The isoelectric point of a physiologically active protein refers to thepH value at which the physiologically active protein has no apparent netcharge in an aqueous solution. The isoelectric point can be determinedin a manner known to those skilled in the art, for example, by means ofisoelectric focusing in which a physiologically active protein iselectrophoresed in solutions of various pH levels to determine the pH atwhich the protein will not migrate. A pH equal to or lower than theisoelectric point of a physiologically active protein is preferably a pHbelow the isoelectric point of the physiologically active protein.

When impurities are DNA molecules in the method of the presentinvention, the pH is preferably adjusted to a level equal to or lowerthan the isoelectric point of a physiologically active protein, so thatthe physiologically active protein is positively charged and the DNAmolecules are negatively charged.

In general, DNA has very strong negative ion charges resulting fromphosphate groups in the backbone (phosphate groups found within stronglyacidic phosphodiester bonds in nucleic acids have a pK value of about1). For this reason, DNA can be negatively charged at any pH and it ispossible to use a desired pH in the range of equal to or lower than theisoelectric point of a physiologically active protein. Since the pHlevel required will vary among different types of physiologically activeproteins, those skilled in the art may select a desired pH level in therange of equal to or lower than the isoelectric point of aphysiologically active protein in a known manner, for example, bypreparing multiple samples with different pHs and measuring theirparameters such as % DNA removal and % protein recovery, as described inthe Example section below. Such a pH is usually pH 2.0 or higher,preferably pH 3.0 or higher, and particularly preferably pH 4.0 orhigher.

To confirm whether DNA molecules are negatively charged, knownprocedures may be used such as those using an electrophoretic titrationcurve (ETC) (see Ion Exchange Chromatography Principles and Methods,Pharmacia (latterly Amersham Biosciences), pp. 52-56).

Moreover, in the method of the present invention, a physiologicallyactive protein-containing sample may also be formed into an acidic oralkaline aqueous solution of low conductivity, followed by adjusting theresulting sample with a buffer to a pH equal to or lower than theisoelectric point of the physiologically active protein.

Thus, in another preferred embodiment of the present invention,impurities in a physiologically active protein-containing sample areremoved by a method comprising the steps of:

1) forming the physiologically active protein-containing sample into anacidic or alkaline aqueous solution of low conductivity;

2) adjusting the resulting sample with a buffer to a pH equal to orlower than the isoelectric point of the physiologically active protein;and

3) removing the resulting particles. Impurities removed by the method ofthe present invention are as described above.

As used herein, the term “acidic aqueous solution of low conductivity”is intended to mean an aqueous solution of pH 2.0 to pH 3.9, preferablyof pH 2.0 to pH 3.0, which has a molarity of 0 to 100 mM, preferably 0to 50 mM, more preferably 0 to 30 mM, or which has an ionic strength of0 to 0.2, preferably 0 to 0.12, or which has a conductivity of 0 to 300mS/m, preferably 0 to 200 mS/m, more preferably 0 to 150 mS/m. Theacidic aqueous solution may be selected from aqueous solutions ofhydrochloric acid, citric acid, acetic acid and other acids. The type,conductivity and pH of acidic aqueous solution of low conductivity willvary depending on the type of physiologically active protein or antibodyto be purified. Those skilled in the art will readily determine optimalconditions for these parameters in preliminary experiments as describedherein.

Likewise, the term “alkaline aqueous solution of low conductivity” asused herein is intended to mean an aqueous solution usually of pH 7.5 topH 13, which has a molarity of 0 to 100 mM, preferably 0 to 50 mM, morepreferably 0 to 30 mM, or which has an ionic strength of 0 to 0.2,preferably 0 to 0.12, or which has a conductivity of 0 to 300 mS/m,preferably 0 to 200 mS/m, more preferably 0 to 150 mS/m. The pH of thissolution will vary depending on the type of physiologically activeprotein or antibody to be purified.

In the method of the present invention, after a physiologically activeprotein-containing sample is formed into an acidic or alkaline aqueoussolution of low conductivity, the resulting sample is adjusted with abuffer to a pH equal to or lower than the isoelectric point of thephysiologically active protein. Examples of a buffer include Tris-HCl,phosphate, Tris, Na₂HPO₄ and NaOH.

Moreover, in the present invention, in certain cases such as where aphysiologically active protein is an antibody, an antibody-containingsample may usually be subjected to affinity chromatography on Protein Aor G and eluted with an acidic aqueous solution of low conductivity,followed by adjusting the resulting eluate with a buffer to a desired pHin the range of equal to or lower than the isoelectric point of thephysiologically active protein.

Thus, in yet another preferred embodiment of the present invention,impurities in a physiologically active protein-containing sample areremoved by a method comprising the steps of:

1) subjecting an antibody-containing sample to affinity chromatographyon Protein A or G and eluting the sample with an acidic aqueous solutionof low conductivity;

2) adjusting the resulting eluate with a buffer to a pH equal to orlower than the isoelectric point of the physiologically active protein;and

3) removing the resulting particles. Impurities removed by the method ofthe present invention are as described above.

The acidic aqueous solution of low conductivity used in this method maybe any of those listed above. Examples of a buffer include Tris-HCl,phosphate, Tris, Na₂HPO₄ and NaOH.

In the method of the present invention, the solution adjusted to a pHequal to or lower than the isoelectric point of the physiologicallyactive protein in the above step, in turn, produces particles (i.e.,becomes clouded). These particles may be removed by filtration through afilter to ensure efficient removal of impurities such as DNAcontaminants. Examples of a filter available for filtration include, butare not limited to, a 1.0-0.2 μm Cellulose Acetate Filter System(Corning) or TFF.

Alternatively, these particles may also be removed by centrifugation orany other techniques for efficient particle removal; procedures forremoval are not limited to filtration through a filter.

Without being bound by any particular theory, the inventors of thepresent invention estimate that when impurities are DNA molecules, eachof these particles is a conjugate formed between physiologically activeprotein and DNA. They also estimate that when the pH is adjusted belowthe isoelectric point of a protein, the protein is positively chargedand DNA molecules are negatively charged, resulting in conjugationbetween DNA and protein. Moreover, the conversion into an aqueoussolution of low conductivity will further enhance conjugation. Particleremoval by filtration results in a small loss of physiologically activeprotein because it is removed in the form of DNA-physiologically activeprotein conjugates. However, such a small loss constitutes only a fewpercent of the total amount of the physiologically active protein; about90% of the physiologically active protein can be recovered, as will bedescribed in the Example section below.

The inventors of the present invention also estimate that Protein A/Gcolumn chromatography alone may not be sufficient to ensure effectiveseparation between DNA contaminant and physiologically active proteinbecause DNA-protein conjugates are formed on the column resin. Thephysiologically active protein thus purified is available for use as apharmaceutical formulation after further purification by cation-exchangechromatography, anion-exchange chromatography, hydroxyapatitechromatography, or combinations thereof.

Quantitative DNA assay may be accomplished by, but not limited to,Threshold Total DNA assay along with DNA extraction prior to the assay.

Quantitative virus assay may be accomplished by, but not limited to,TCID₅₀ (tissue culture infective dose (50%)) assay which is measured byviral infectivity in detection cells, in combination with RT/Q-PCR andQ-PCR which allow determination of the virus amount in fractions.

The present invention will now be further described in the followingexamples, which are not intended to limit the scope of the invention.Based on the detailed description, various changes and modificationswill be apparent to those skilled in the art, and such changes andmodifications fall within the scope of the invention.

EXAMPLES Example 1 Investigation of Buffer Composition for Protein AAffinity Chromatography in Purifying hPM-1 (Humanized Anti-IL-6 ReceptorAntibody)

1.1. Test Procedures

(1) Test Material (Antibody-Containing Sample)

A sample containing the culture medium (hereinafter abbreviated as CM)of CHO cells producing hPM-1 antibody (humanized anti-IL-6 receptorantibody), which had been centrifuged to remove the cells and stored at−80° C., was filtered through a 0.22 μm Cellulose Acetate (abbreviatedas CA) Filter System (CORNING) and used as a test sample forpurification investigation. The hPM-1 antibody was prepared as describedin Reference Example 2 of JP KOKAI 08-99902 using the human elongationfactor Iα promoter shown in Example 10 of International Publication No.WO92/19759 (isoelectric point: pH 9.0).

(2) Instrument Used for Examination

For HCl Eluate

-   HPLC: L-6200 Intelligent Pump (HITACHI) L-4200 UV-VIS Detector    (HITACHI) D-2500 Chromato-Integrator (HITACHI)-   Column: HR5/2 (Pharmacia), 5 mm I.D.×20 mm H-   Media: POROS 50A (PerSeptive), 0.4 ml Lot; A250-039, Code; SPECIAL    For Particles-   HPLC: Waters PrepLC4000 System (Waters) Waters2000 System Controller    (Waters) Waters486 Tunable Absorbance Detector (Waters) Waters741    Data Module (Waters)-   Spectrophotometer: U-2000 (HITACHI)-   Column: XK26 (Pharmacia), 26 mm I.D.×100 mm H-   Media: POROS 50A (PerSeptive), 53 ml Lot; A250-039, Code; SPECIAL    (3) Analysis and Assay-   hPM-1 Assay:

hPM-1 is assayed by reversed-phase HPLC on a PLRP-S column (PolymerLaboratories) with a linear gradient.

-   DNA Assay:

DNA is measured by Threshold Total DNA assay. Prior to the assay, DNAextraction is performed (e.g., using a DNA extracter kit, Wako PureChemicals Industries, Ltd.). Likewise, a Threshold Total DNA assay kit(Molecular Devices) is used for the measurement.

-   Turbidimetry:

Each test sample is monitored for particle formation by measuring itsabsorbance at 660 nm in a spectrophotometer U-2000 (HITACHI).

1.2. Investigation of Elution Conditions

Elution conditions were investigated at various buffer compositions forelution in Protein A affinity chromatography by measuring % recovery ofhPM-1 and DNA removal by elution. The above antibody-containing samplewas subjected to the column under the conditions indicated in Table 1below. Protein A resin was equilibrated with the equilibration bufferindicated in Table 1 and then loaded with the above antibody-containingsample, followed by Washing 1, Washing 2 and elution. The elutionprofile was monitored at A280 nm to isolate a protein peak. In thetable, C-P Buffer denotes citrate-phosphate buffer.

TABLE 1 Elution method 1 Elution method 2 Elution method 3 Equilibration1M NaCl/100 mM C-P 1M NaCl/10 mM C-P 1M NaCl/100 mM C-P Buffer pH7.5Buffer pH7.5 Buffer pH7.5 Washing 1 1M NaCl/100 mM C-P 1M NaCl/10 mM C-P1M NaCl/100 mM C-P Buffer pH7.5 Buffer pH7.5 Buffer pH7.5 Washing 2 100mM C-P Buffer pH7.5 10 mM C-P Buffer pH7.5 100 mM C-P Buffer pH7.5Elution 100 mM C-P Buffer pH2.6 2.5 mM HCl pH2.6 2.5 mM HCl pH2.6

No chromatographic difference was observed among Elution methods 1, 2and 3.

Each elution fraction was adjusted to pH 7.0 with a 300 mM Trissolution, indicating that particles were generated in the fractionseluted with HCl (Elution methods 2 and 3). Further investigation wasperformed to determine the correlation between particle formation and %recovery of hPM-1 or the amount of residual DNA.

To examine the particle correlation, the HCl eluate from Elution method2 was supplemented with NaCl and analyzed for the correlation betweenNaCl concentrations (0 mM, 50 mM, 100 mM) and various factors. In theanalysis of the correlation between NaCl concentrations and variousfactors, filtered and unfiltered samples were prepared as follows: eachProtein A elution fraction supplemented with NaCl was adjusted to pH 7.0with a 300 mM Tris solution and then filtered or unfiltered through a0.22 μm CA Filter. The filtered and unfiltered samples were measured for% recovery of hPM-1 (filtered samples only) and the amount of residualDNA.

1.3. % Recovery

The % recovery of hPM-1 was measured for the individual elution methods.As a result, the % recovery was as high as 98.6% in Elution method 1. Incontrast, the % recovery ranged from 83.8% to 97.1% in Elution method 2and from 83.5% to 93.7% in Elution method 3; these variations wereestimated to be due to the smallness of examination scale (resin volume:0.4 ml). When the purification scale was increased, it was confirmedthat the % recovery of hPM-1 was stabilized at 90% or more (Elutionmethod 2). Thus, the % recovery of hPM-1 was also found to remain higheven in HCl elution.

1.4. Correlation Between NaCl Concentrations in the HCl Eluate andVarious Factors

Table 2 summarizes the analysis of the correlation between NaClconcentrations in the HCl eluate and various factors.

TABLE 2 NaCl concentration 0 mM 50 mM 100 mM Turbidity (pH unadjusted)0.004 0.007 0.011 Turbidity (pH adjusted) 0.252 0.049 0.020 % Recoveryof hPM-1 (filtered) (%) 81 86 88 Amount of DNA (unfiltered) 98 220 241(pg DNA/mg hPM-1) Amount of DNA (filtered) 11 30 250 (pg DNA/mg hPM-1)

For the filtered samples, the % recovery of hPM-1 was 88% at 100 mMNaCl, 86% at 50 mM NaCl and 81% at 0 mM NaCl. The amount of residual DNAwas low at 0 mM NaCl in both filtered and unfiltered samples. Inparticular, the filtered sample supplemented with 0 mM NaCl had a verylow DNA content of 11 pg DNA/mg hPM-1.

The pH-adjusted samples with a higher turbidity tend to provide a lower% recovery of hPM-1 and a smaller amount of residual DNA afterfiltration. This result suggests a high possibility that hPM-1 and DNAboth contribute to particle formation. It is estimated that hPM-1 andDNA probably interact with each other to form particles by adjusting thepH to 7.0. In view of achieving a higher % recovery of hPM-1, it ispreferable to increase the NaCl concentration in the HCl eluate. In viewof decreasing an amount of residual DNA, on the other hand, it isdesirable to eliminate NaCl supplementation into the HCl eluate.

Example 2 Purification of Humanized Anti-PTHrP Antibody

A sample containing a humanized anti-PTHrP antibody (a culture mediumfrom CHO cell culture, filtered through 0.45 and 0.2 μm CA SARTOBRAN Pfilters (sartorius)) was purified by Protein A affinity columnchromatography under the conditions indicated below. The anti-PTHrPantibody was prepared as described in International Publication No.WO98/13388 (isoelectric point: pH 8.3).

2.1. Experimental Conditions

-   Purification apparatus: AKTA explorer (Amersham Pharmacia Biotech)-   Column: HR5/5, C10, XK-26 (Amersham Pharmacia Biotech)-   Resin: rprotein A Sepharose Fast Flow-   Load: direct load of the culture medium (pH 6.6 to pH 7.5)-   Adjustment of elution fraction: elution fractions are adjusted to    various pH levels with a 1 M aqueous Tris solution and then filtered    through a 0.2 μm Cellulose Acetate (hereinafter abbreviated as CA)    to remove DNA (the conditions are examined in (1) below).

The Protein A column was sufficiently equilibrated with 150 mMNaCl-containing citrate-phosphate buffer (pH 7.5) and then loaded withthe above antibody-containing CM. Subsequently, the column was washedwith 150 mM NaCl-containing citrate-phosphate buffer (pH 7.5) to removeunbound impurities, further washed with citrate-phosphate buffer (pH7.5) to decrease the conductivity, and then eluted with 20 mM aqueouscitric acid. The elution profile was monitored at A280 nm to isolate aprotein peak. This Protein A elution fraction was used for the followingexamination of conditions.

2.2. Examination of Removal Conditions for Residual DNA in the Eluate

To ensure efficient removal of residual DNA, the optimal pH forfiltration through a filter was investigated. The Protein A elutionfraction was adjusted with a 1.0 M aqueous Tris solution to thefollowing pH levels: 2.7 (unadjusted), 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0and 7.5. Subsequently, each sample was allowed to stand for a givenperiod of time, filtered through a 0.22 μm CA filter, and then adjustedto pH 7 with a 1.0 M aqueous Tris solution, followed by DNA assay. Table3 lists the examined pH levels and standing periods, along with theamount of residual DNA.

TABLE 3 Removal of residual DNA (unit: pg/mL) Direct pH 7.5 pH 7.0 pH6.5 pH 6.0 pH 5.5 pH 5.0 pH 4.5 pH 4.0 (pH 2.7)  0 hr. 984 83.3 53.8<22.5 <15.0 17.2 54.1 32,052 40,878  6 hr. 816 51.9 <15.0 <22.5 <15.0<15.0 44.0 38,172 42,078 24 hr. 310 46.6 <15.0 <22.5 <15.0 <15.0 39.742,528 30,222 (DNA in the culture medium: 6,637,200 pg/mL; DNA in theunfiltered sample: 25,110 pg/mL)

As shown in the table, the amount of residual DNA was below thedetection limit at pH 5.5 and pH 6.0 in all cases where the samples wereallowed to stand for 0, 6 and 24 hours. Also, the removal of residualDNA reached a peak around pH 5.5 and pH 6.0, whereas decreasedefficiency of DNA removal was observed at higher and lower pH levels.

Example 3 Purification of Humanized Anti-HM1.24 Antigen MonoclonalAntibody

A sample containing a humanized anti-HM1.24 antigen monoclonal antibody(a culture medium from CHO cell culture) was purified by Protein Aaffinity column chromatography under the conditions indicated in Table 4below. The anti-HM1.24 antigen monoclonal antibody was prepared asdescribed in International Publication No. WO98/14580 (isoelectricpoint: pH 9.0).

3.1. Experimental Conditions

-   Column: rprotein A FF, 5 mL (16 mm ID×25 mm H)-   Flow rate: 5 mL/min (150 cm/h)-   Sample: direct load of the culture medium

TABLE 4 Equilibration (20 CV) 10 mM C-P Buffer, 1 M NaCl, pH 7.5 LoadDirect load of CM Washing 1 (20 CV) 10 mM C-P Buffer, 1 M NaCl, pH 7.5Washing 2 (20 CV) 10 mM C-P Buffer, pH 7.5 Elution (10 CV) Citric acid,pH 2.5 Washing 3 (4 CV) 0.1 M NaOH

The Protein A column was sufficiently equilibrated with 150 mMNaCl-containing citrate-phosphate buffer (pH 7.5) and then loaded withthe above antibody-containing CM. Subsequently, the column was washedwith 150 mM NaCl-containing citrate-phosphate buffer (pH 7.5) to removeunbound impurities, further washed with citrate-phosphate buffer (pH7.5) to decrease the conductivity, and then eluted with 20 mM aqueouscitric acid. The elution profile was monitored at A280 nm to isolate aprotein peak. This Protein A elution fraction was used for the followinginvestigation of conditions.

3.2. Investigation of Removal Conditions for Residual DNA in the Eluate

To ensure efficient removal of residual DNA, the optimal pH forfiltration through a filter was investigated. The Protein A elutionfraction was adjusted with a 1.0 M aqueous Tris solution to thefollowing pH levels (pH=4.5-7.5). Subsequently, each sample was allowedto stand for a given period of time, filtered through a 0.22 μm CAfilter, and then adjusted to pH 7 with a 1.0 M aqueous Tris solution,followed by DNA assay and reversed-phase HPLC for assay of the humanizedanti-HM1.24 antigen monoclonal antibody. Table 5 shows the results ofDNA assay, while Table 6 shows the yield of the humanized anti-HM1.24antigen monoclonal antibody.

TABLE 5 Removal of residual DNA (unit: pg/ml) Experiment 1 pH 7.5 pH 6.5pH 5.5 0 h 1142 624 113 6 h 3288 1157 117 (DNA in the culture medium:235200 pg/ml) Experiment 2 pH 5.5 pH 5.0 pH 4.5 0 h 137 67 86 6 h 94 34164 (DNA in the culture medium: 5448000 pg/ml; DNA in the unfilteredsample: 4330 pg/ml)

TABLE 6 % Recovery of humanized anti-HM1.24 antigen monoclonal antibodyby filtration pH 5.5 pH 5.0 pH 4.5 0 h 98.1% 89.6% 87.8% 6 h 89.3% 91.1%98.6%

Although the samples purified by Protein A affinity chromatography werestill rich in DNA, Experiment 1 indicated that the amount of DNAdecreased with decrease in pH in the order of pH 7.5, pH 6.5 and pH 5.5,and that there was a tendency to remove more DNA at 0 hours than at 6hours. In Experiment 2, the same experiment was carried out underconditions of pH=4.5, 5.0 and 5.5, indicating that DNA was sufficientlyremoved to the same extent, regardless of pH and standing period withinthe tested range. In addition, the calculation of % recovery indicatedlittle loss of the humanized anti-HM1.24 antigen monoclonal antibody.

Example 4 Purification of Granulocyte Colony-Stimulating Factor (G-CSF)

A G-CSF-containing sample (from CHO cell culture; Chugai PharmaceuticalCo., Ltd.) was used for the following examination of conditions(isoelectric point: pH 5.5-5.7).

4.1. Investigation of Removal Conditions for Residual DNA in the Eluate

To ensure efficient removal of residual DNA, the optimal pH forfiltration through a filter was investigated. The G-CSF-containingsample was diluted in an acidic solution of low conductivity (2.5 mMaqueous HCl) and further formed into an acidic aqueous solution of lowconductivity using 20% hydrochloric acid, followed by addition of sampleDNA. The G-CSF-containing sample thus treated was adjusted with a 1.0 Maqueous Tris solution to the following pH level (pH=4.3 or 6.6) and thenfiltered through a 0.22 μm CA filter. Subsequently, DNA assay wasperformed on both filtered and unfiltered fractions. Table 7 shows theresults of DNA assay.

TABLE 7 Removal of residual DNA (unit: pg/ml) pH for filtration pH 6.6pH 4.3 Unfiltered 4.3 × 10⁵ 4.3 × 10⁵ Filtered 2.8 × 10⁴ <90

This investigation confirms efficient reduction of DNA in theG-CSF-containing sample rich in DNA when the sample was filtered at pH4.3; namely, the amount of residual DNA was below the assay limit ofdetection.

Example 5 Effects of Virus Removal on the Purification of hPM-1(Humanized Anti-IL-6 Receptor Antibody)

5.1 Test Material (Antibody-Containing Sample)

Samples containing the culture medium (CM) of CHO cells producing hPM-1antibody (humanized IL-6 receptor antibody), which had been centrifugedto remove the cells and stored at −80° C., were supplemented withX-MuLV, Reo3 and MVM, respectively, followed by filtration through a0.45 μm filter (Bottle Top Filter, CORNING) for use as test samples forpurification investigation. The hPM-1 antibody was prepared as describedin Example 1. The viruses used for examination were each obtained fromATCC (American Type Culture Collection).

5.2 Purification by rprotein Column Chromatography

The virus-supplemented samples prepared in 5.1 were purified by rproteinColumn Chromatography. Detailed conditions are as shown below.

Resin: rProteinA Sepharose Fast Flow

Instrument: AKTA explorer100, AKTApurifier

Column: XK16/20, XK16/40

Resin height: 11.5 cm

Elution conditions

-   -   Equilibration: 1 mol/L-NaCl, 20 mmol/L C-P Buffer, pH 7.5±0.2,        Conductivity 8.5±0.5 S/m    -   Washing 1: 1 mol/L NaCl, 20 mmol/L C-P Buffer, pH 7.5±0.2,        Conductivity 8.5±0.5 S/m    -   Washing 2: 10 mmol/L C-P Buffer, pH 7.7±0.2, Conductivity 165±20        mS/m    -   Elution: 2.5 mmol/L HCl, pH 2.7±0.2, Conductivity 107±10 mS/m        5.3 Low pH Treatment

The elution fractions obtained in 5.2 were adjusted to pH 3.2±0.1 with 1mol/L hydrochloric acid and held at a room temperature of 15±5° C. for30 minutes or longer. Subsequently, each elution fraction was adjustedto pH 7.2±0.1 with a 300 mmol/L Tris solution, 40.0 mL of which was thenfiltered under a pressure of 0.03±0.01 MPa using a filtration unitequipped with a glass fiber filter (Millipore)(0.2 μm, PALL) connectedto the primary side and a BioInert (0.2 μm, PALL)(a PALL filter holderequipped with a φ15 mm adjuster) connected to the secondary side.

5.4 Detection of Viruses

All the samples collected were measured by TCID₅₀ assay. In theclearance capacity test for X-MuLV and MVM, these viruses were detectednot only by TCID₅₀ assay which was measured by viral infectivity indetection cells, but also by RT/Q-PCR and Q-PCR which alloweddetermination of the virus amount in fractions.

5.5 Results

The results of detection in 5.4 are shown in the tables below.

TABLE 8 Virus titer (TCID₅₀ assay: Log₁₀/mL) Reo3 MVM Run 1 Run 2 Run 1Run 2 Unfiltered 5.76 5.76 4.80 4.18 Filtered ≦1.03 ≦1.03 ≦1.03 ≦1.03

TABLE 9 Virus titer (PCR: Log₁₀ Copies/5 μL) X-MuLV MVM Run 1 Run 2 Run1 Run 2 Unfiltered 5.05 4.77 4.18 2.83 Filtered ≦1.90 ≦1.90 ≦1.30 ≦1.90

As shown above, the purification process of the present inventionachieves very high LRVs (Logarithmic Reduction Values) for all thetested viruses and this examination confirms that the viruses wereremoved to a level below the assay limit of detection after low pHtreatment and filtration.

INDUSTRIAL APPLICABILITY

The method of the present invention enables efficient removal ofimpurities such as DNA contaminants and viruses in a very simple manner,and is significantly advantageous in purifying physiologically activeproteins, especially antibodies. The method achieves an extremely lowDNA concentration (e.g., 22.5 pg/ml) when impurities are DNA molecules,while it achieves an extremely low virus titer (e.g., 1.03 (expressed inLog₁₀/mL), as measured by TCID₅₀ assay) when impurities are viruses. Themethod of the present invention also enables cost reduction and hasgreat significance in this field.

The invention claimed is:
 1. A method for removing DNA contaminants froma genetically produced antibody-containing sample, which consists of thesteps of: 1) applying the sample to a protein-A or protein-G affinitycolumn, which binds the antibody, 2) washing the column to elutenon-bound components of the sample, 3) eluting the bound antibody withan aqueous solution of pH of 2.0 to 3.9 and an ionic concentration of 50mM or less, thereby forming an acidic aqueous solution of the antibodyhaving an ionic concentration of 50 mM or less and a pH of 2.0 to 3.9,4) forming particles containing the DNA contaminants by a stepconsisting of adjusting the pH of the eluate solution to a pH of from4.0 to 7.5, and 5) removing the particles of step 4) wherein the sampleis a culture medium from an antibody-producing cell culture.
 2. Themethod according to claim 1, wherein the particle-containing solution ofstep 4) has a conductivity of 300 mS/m or less.
 3. The method accordingto claim 1, wherein the acidic aqueous solution for eluting the sampleof step 2) is selected from aqueous solutions of hydrochloric acid,citric acid and acetic acid.
 4. The method according to claim 1,wherein, after the removal of DNA particles, the DNA contaminants are ata DNA concentration of 22.5 pg/ml or less.
 5. The method according toclaim 1, wherein the particles are removed by filtration through afilter.
 6. The method according to claim 1, wherein the buffer is anaqueous solution of Tris.
 7. The method of claim 1, wherein the antibodyis a chimeric antibody.
 8. The method of claim 1, wherein the antibodyis a humanized antibody.